Many applications, such as computer power supply and power supplies in TV and Video sets, require low voltage DC output power for use by analog and digital circuitry. However, the power available to them is the mains power which is high voltage AC, supplied by an AC electric power utility and usually within the range of 80 VAC and 600 VAC. As the mains power is the only power available for use with these types of applications the high voltage AC mains power requires to be converted to low voltage DC power before supplying to the components.
The available power supply systems, to provide the high voltage AC to low voltage DC conversion, can be broadly classified into four categories: the mains frequency transformer approach, the high voltage linear regulator approach, the high voltage capacitive coupling approach, and the switching power supply approach.
The transformer-based power supplies approach uses a step down mains frequency transformer and some type of wave rectification. These power supplies are isolated from the mains power supply but this isolation requires a bulky and expensive transformer. Further, size of other components, such as capacitors, that are used in conjunction also increases due to the low frequency of operation (50/100 Hz or 60/120 Hz).
The high voltage linear regulator approach eliminates the large, costly step down mains frequency transformer, but has the disadvantage of large capacitors (due to the low frequency of 50/100 Hz or 60/120 Hz) and high power dissipation requirements because the excess voltage has to be dropped across the linear pass element.
The high voltage capacitive coupling power supplies approach also eliminates the step down transformer and has better efficiency than the high voltage linear regulator approach but has poor regulation and requires large high voltage capacitive elements.
The available switching power supplies approach can be further classified into three classes. In the first class are the conventional switching power supplies that can step down high voltage AC from mains power supply to low voltage DC with a very small transformer because of the high switching frequency. These power supplies are also isolated from the mains but the transformer and switch element must be able to withstand the mains voltage and switching transients. Further, the filter capacitors at the input to these switching power supplies must be rated to withstand the maximum peak line voltage and are required to have enough capacitance to maintain the voltage ripple within acceptable limits at the minimum line voltage. These two conditions result in physically large capacitors. These requirements increase the cost and size, though not to the level of the linear power supplies, and make it difficult to use in space-constrained applications, such as telemetry modules for smart electric utility meters, computers, and TV sets.
For example, FIG. 1 is a diagram of a conventional switching power supply used to convert the AC line voltage 110 and produce DC output voltage 170. The power supply includes a bridge rectifier 120 and a DC-DC converter 100. It will be understood by those skilled in the art that the filter capacitor 130, the switch 140, and the transformer 150 all must be rated to withstand the peak of the maximum input voltage 110 with an adequate margin of safety. For example, for 600 VAC input (480 VAC with 25% safety margin) the rating is 848.5V. Thus, the filter capacitor 130, the switch 140, and the transformer 150 must be capable of withstanding 848.5V plus any switching transients that may be generated.
In the second class are the switching power supplies that produce low voltage DC from high voltage AC supplied from mains power supply by using a switch that turns on when the input voltage is below the desired output voltage and turns off when this threshold is exceeded. These are now commercially available as single chip solutions with an external switch. FIG. 2 illustrates such a switching power supply which rectifies and regulates high voltage alternating current without the use of transformers, large capacitive coupling circuits, or high voltage linear regulators. The device includes a rectifier 220, a control circuit 230 for sensing the output voltage of the rectifier 220 and switching on and off the output of rectifier 220, a first storage capacitor 240, a low voltage linear regulator 250 and a second storage capacitor 260. The control circuit 230 effectively divides the device into a high voltage subsystem 200 and a low voltage subsystem 280. Although these devices provide advantages in terms of low cost and smaller size, the disadvantages are that they are not isolated from the mains power supply and the linear regulator drastically reduces the efficiency if there is any significant difference between the output voltage of the control circuit 230 and the final output voltage 270.
FIG. 3A through FIG. 3D illustrate a voltage waveform at different points in the circuit of FIG. 2. As shown in FIG. 3A, the voltage waveform 310 of the output of the rectifier 220 to the control circuit 230, is a rectified form of the input voltage 210 at the same magnitude as the input voltage 210. The typical output from control circuit 230 for such an input from the rectifier 220 would be the voltage waveform 320 as shown in FIG. 3B, in which the circuit is closed whenever the full wave rectified voltage is below a prescribed threshold voltage 300, for example 40 Volts. However, the waveform 330 in FIG. 3C shows how the output of the control circuit 230 is altered due to the presence of capacitor 240 in the circuit design of FIG. 2. The low voltage linear regulator 250 of FIG. 2 then produces the regulated DC output voltage waveform 340 as shown in FIG. 3D, though at a limited output power as noted above.
In the third class are the switching power supplies that are a combination of switching power supplies of first and second classes. These use the switching power supply of second class as a pre-regulator for the switching power supply of first class. This results in a power supply that is low cost and compact and is isolated from the high voltage AC mains power supply, but needs two separate switches. The first switch is a high voltage low frequency switch and it acts as a pre-regulator to a second low voltage high frequency switch that does the DC-DC conversion. The second switch may be part of an off the shelf “Brick” DC-DC converter.
Such a device is shown in FIG. 4, and includes a rectifier 420 for receiving a high voltage AC line power input and for outputting a full wave rectified, high voltage DC, a gating component 430 coupled to the rectifier 420 for receiving the high voltage full wave rectified DC output, acting as the high voltage low frequency switch and outputting an intermediate voltage DC capped by a preset voltage threshold, a first capacitor 440 to smooth out AC ripples, a DC-DC converter 450 coupled to the gating component 430, for receiving the intermediate voltage DC output, through the first output capacitor 440, wherein the DC-DC converter 450 is configured to step down the intermediate voltage DC to a desired high current, low voltage DC output using the second low voltage high frequency switch 460 integrated into the DC-DC converter 450 and a second capacitor 470 coupled to the output of the DC-DC converter 450 to further smooth out the high current, low voltage DC output.
FIG. 5A through FIG. 5D illustrate a voltage waveform at different points in the circuit of FIG. 4. As shown in FIG. 5A, the bridge rectifier 420 rectifies the AC input voltage 410, which may range from 80 to 600 VAC, and provides the full wave rectified DC waveform 510. Now, the gating component 430 turns on at zero crossing and turns off when the full wave rectified DC voltage exceeds a preset voltage threshold VT (shown as threshold 500 in FIG. 5A through 5D), allowing an intermediate DC voltage. Next, the capacitor 440 reduces the AC ripples from the intermediate DC voltage and provides a pre-regulated intermediate DC voltage 530 to the DC-DC converter 450, including switch 460 and the transformer, as shown in FIG. 4. These components step down the pre-regulated intermediate voltage DC 530, with another capacitor 470 to further reduce the AC ripples, to a predetermined final DC voltage 540, as shown by curve 540 in FIG. 5D.
The third type of switching power supply is an improvement over the second type because it replaces the linear regulator 250 of FIG. 2 with a DC-DC converter 450 in FIG. 4, and thus improves the current output capability and efficiency. The need for two separate switching elements (high voltage, low frequency switch used for gating and low voltage high frequency switch used for DC-DC conversion) is a disadvantage because it adds cost and complexity.
There is therefore a need for improved systems, devices, and circuit designs for converting high voltage AC to low voltage DC without the use of large high voltage filter capacitors or large high voltage switching power supplies or multiple switches, while also providing for high current low voltage DC outputs. Further, there is a need to provide methods, systems, circuit designs, and devices to reduce the size and cost of a power supply module.